How do you extract semen from sharks? Carefully!

Australian marine scientists are using an ingenious, if peculiar method to extract semen from wild male leopard sharks.

It isn’t an experiment. It’s a bid to boost the species’ numbers and genetic diversity through artificial insemination.

The technique relies on “tonic immobility”, also known as animal hypnosis, a reflex seen in a wide variety of vertebrates and invertebrates alike.

“Never thought I’d be getting this intimate with sharks underwater,” Dr Christine Dudgeon of the University of the Sunshine Coast joked in response to photos of her team using the technique for the first time last December.

“Our 5-person team could syringe out semen and blood samples from male sharks underwater in the wild using ‘tonic immobility,’ where sharks go into a sleep-state on their backs,” she explains.

Tonic immobility causes the animal to go into a temporary state of inactivity. According to Shark Trust, this “trance-like state” can occur naturally in sharks and can also be induced by stimulating the tiny sensory pores located on their snouts.

The Great Australian Stegostoma Semen Expedition (GASSE) used the reflex to immobilise and collect semen from male leopard sharks gathering off North Stradbroke Island near Brisbane. The same technique was then successfully used to artificially inseminate female leopard sharks in aquariums across Australia and Singapore.

“Once these eggs are laid and veterinarians have determined they are fertile, they will be sent to our partners in the Raja Ampat Islands, off West Papua, until they hatch into juveniles that will hopefully help restock those protected waters,” says Dudgeon.

The method was pioneered by Dr Paolo Martelli, director of veterinary services at Ocean Park Hong Kong.

Leopard sharks (Stegostoma tigrinum) are also known as zebra sharks because the pups have stripes that eventually develop into spots. They are found in coral reefs and sandy flats up to 62m deep in tropical waters throughout the Indo-Pacific.

They grow to more than 2m in length and have a flat belly that allows them to swim close to the sea floor. Leopard sharks pose no threat to humans, spending most of the day resting motionless at the bottom of the ocean and becoming active at night to hunt molluscs, crustaceans, and small fish.

“They are not threatened in Australia because we don’t fish them,” says Dudgeon.

“In other countries they have been over-harvested for fins and meat. Their beautiful, very tough skin has become prized by skin traders for use in the wall linings of expensive yachts.”

The global population has become severely fragmented as a result and is listed as endangered on the IUCN Red List of Threatened Species.

“We hope this marine reproduction technique will be a game-changer for international projects aiming to replenish the Stegostoma species globally, particularly in areas such as Indonesia where it is in danger of becoming extinct,” says Dudgeon.

Her team will dive off northern New South Wales in late March to collect further semen samples, where they will also insert acoustic tags to track the animals.

“We can now follow these sharks’ movements via a network of marine acoustic receivers to further inform conservation work for this ambitious restocking project.”